Preparation of bacterial cultures The bacterial cultures were grown either from fire blight in fected branches or from previously stored strains as glycerol stocks at −80 C. For bacterial isolation and culture from in fected branches, approximately 2.5 cm of stem tissue sur rounding the visual symptoms of fire blight was used. Material was surface sterilized with 70% ethanol for 1 min, and treated afterwards with 10% Clorox bleach for 10 min.